1. Field of the Invention
The present invention relates to cooling rack-mounted computer equipment, and more particularly to the detection of hot air recirculation and other airflow anomalies.
2. Description of the Related Art
An airflow anomaly is one type of fault scenario that may be experienced by a rack-mounted computer system. In the present context, an airflow anomaly may be defined to include any unexpected or non-ideal airflow condition affecting a computer system. Some airflow anomalies result in higher than expected air temperatures in a computer system. These elevated temperatures may prevent proper cooling of components, causing them to heat up more than expected and possibly exceed safe operating temperatures.
Elevated intake temperatures can occur when heated air from a first component rack is exhausted to a second component rack. The heated air exhausted from the first component rack may have detrimental effects on the cooling of the second rack. To avoid this complication, component racks may be arranged according to a “hot aisle” and “cold aisle” layout, whereby neighboring component racks intake air from a shared cold aisle and exhaust air to a common hot aisle. This arrangement helps minimize the likelihood of appreciable volumes of heated exhaust air from entering a neighboring component rack. If a data center includes more than one row of component racks, then there may be alternating hot and cold aisles with a component rack between each aisle. The orientation of the component racks is also alternated in that case, so that the intake side of each component rack faces a cold aisle and the exhaust side of each component rack faces a hot aisle.
Another, more common cause of hot air entering a component rack is the recirculation of heated air (“hot air recirculation”). Hot air recirculation (which may alternatively be referred to simply as “recirculation”) is a phenomenon wherein hot exhaust air from a rack flows back to the air intake of the same rack without first being cooled as intended. Recirculation may be caused, for example, by an obstruction, such as a wall near the rack, which causes the exhaust air to flow around the side of a rack. Another condition that can cause recirculation is the insufficient rejection of hot air from behind a component rack, resulting in a heat plume which extends over the top and front of the rack and returns to the air intake of the component rack without having been sufficiently cooled. Still another cause of recirculation is the occurrence of gaps between the components in a component rack through which hot air is drawn due to the pressure differential between the front and rear panel of the rack.
Recirculation has been observed to increase intake air temperatures by 6 to 8 degrees Celsius in some installations. Recirculation can, therefore, negatively affect the cooling of a component, causing the components in a rack to run hotter than desired. This may result in shorter “Mean Time Between Failure” (MTBF), increased energy consumption due to increased fan speed, inefficient use of cool air, increased leakage current, and reduced thermal bandwidth for emergency operations. The additional heat introduced to the rack by recirculating air may also cause the components to exceed thermal thresholds, particularly if the components are operating in a room that is very warm to begin with, or if the amount of heating that takes place within the components is substantial. These elevated temperatures can cause one or more of the components to shut down or require them to be removed from service.